During radiation therapy of human tumors, the majority of DNA lesions are produced from the radiolysis of water in the vicinity of the DNA molecule. Whether or not these lesions block DNA replication and lead to cell death and therapeutic efficacy or are bypassed resulting in mutations and the possibility of secondary tumors at the site of treatment merits significant consideration. There is a choreography of events that ensue following tumor treatment by ionizing radiation. The damages may be repaired, thus the cancer cells survive. The damages may remain in the DNA molecule, block DNA replication and initiate a cascade of signals leading to cancer cell death. In adjacent irradiated normal cells, unrepaired damages may be bypassed, leading to mutation and potentially secondary cancers. Thus, it is the interaction between an unrepaired lesion and a DNA polymerase that determines the ultimate fate of the cell. In order to maximize therapeutic gain and minimize the formation of second tumors at the same site, it is critical to have a mechanistic understanding of the interaction between DNA polymerases and radiation-induced DNA lesions. The proposed study addresses this question with respect to both DNA polymerase blocking and potentially mutagenic radiation-induced DNA damages. Sites of base loss and thymine glycols are frequently produced and replication blocking DNA damages while 5-hydroxycytosine, 8-oxoguanine and 2-hydroxyadenine are commonly produced premutagenic lesions. All are major lesions produced by ionizing radiation and their structure/function interactions will be studied in the context of replicative and bypass DNA polymerases. This research bears directly on public health since it addresses the fundamental mechanisms underpinning cell killing and mutagenesis by therapeutic levels of ionizing radiation.